The long term objectives of this proposal are unified by the overriding hypothesis that the occurrence of postflight orthostatic hypotension in some, but not all, astronauts is contributed to by gender-related differences in autonomic regulation of arterial pressure which precipitates orthostatic hypotension in predisposed individuals. The purpose of this study is to test the hypothesis that greater orthostatic intolerance in women is caused by diminished vasoconstrictive responses that are estrogen related. A ground based study will include pre menopausal women at different phases of their menstrual cycle; postmenopausal women before and after estrogen replacement; and men. All groups will undergo an upright tilt test before and after cardiac blockade to test the relative susceptibilities of those who are primarily heart rate responders and those who are primarily vascular responders. Levels of estrogen, catecholamine responses, and alphaadrenergic receptors will be measured as well as cardiovascular responses. To date, studies have been completed in four men and three menopausal women.